Shift work forces individuals to be awake and perform at a time when biological sleep propensity is high and conversely to sleep when sleep propensity is minimal. This misalignment induces poor sleep and impaired alertness and performance. In addition, irregular light-dark cycles as a consequence of rotating between diurnal and nocturnal work schedules can induce circadian disruption (James et al., 2004; Santhi et al., 2005). Circadian and sleep disruption are linked to adverse health effects, and epidemiological studies have demonstrated a strong association between shift work and adverse health effects ranging from mood disorders to various forms of cancer (Harrington et al., 2001).
Different approaches have been tested for improving sleep and performance in shift workers. These include acute solutions such as the use of hypnotics to facilitate daytime sleep and/or the use of stimulants to improve performance during night work. Another approach involves inducing either partial (Lee et al., 2006; Smith et al., 2009) or complete (Czeisler et al., 1990; Horowitz et al., 2001; Santhi et al., 2005) circadian phase realignment such that the endogenous circadian phase for sleep and alertness correspond appropriately to external sleep and work schedules respectively. Since light is the strongest circadian resetting cue, a regimen of exposure to bright light during night work and darkness during scheduled daytime/evening sleep can be used to treat physiological maladaptation to night work. While complete inversions are the most effective in improving alertness, performance and sleep, they may be difficult to induce in a home-based setting and a complete inversion of circadian rhythms is seldom achieved, even across successive night shifts and after years of night work experience (Folkard et al., 2008). One of the practical difficulties with maintaining inverted circadian rhythms is exposure to bright light at times when individuals have to remain in darkness, for example exposure to sunlight during morning commutes or having to meet daytime social obligations (Crowley et al., 2003; Sasseville et al., 2009). In addition, it may be difficult to induce large (12-hour) phase shifts rapidly to completely invert rhythms when there is limited time between day and night shifts.
Recent work using simulated night shift paradigms suggest that a compromise circadian phase position, such that the highest circadian propensity for sleep occurs in the morning (˜1000 h), may improve performance during night shifts and improve sleep during the daytime after night shifts and during the late-night on non-working days (Lee et al., 2006). In order to attain such a compromise phase position, individuals were exposed to four or five bright light (˜4100 lux) pulses during their simulated night shifts, used dark sunglasses (15% average transmission; 0% below 400 nm) during daytime hours while they were awake, and had scheduled sleep episodes during the day (while on night shifts). This approach of achieving a compromise-phase position involves the use of dark sunglasses to minimize the resetting effects of light when an individual is awake but needs to remain in darkness to prevent unwanted circadian phase resetting.
An alternate approach exploits the differential sensitivity of the circadian pacemaker to short-wavelength light for photic phase resetting (Brainard et al., 2001; Thapan et al, 2001). Recent work has demonstrated that using glasses that attenuate short-wavelengths (less than 25% transmission <530 nm) during the morning and restrict light exposure at nighttime improved sleep in permanent night shift workers (Sasseville et al., 2009).